The thing is, the only way to realize the STABILIZATION and ALIGNMET modes of the MiG-21 AP is to involve a lot of trimming. The Modes are modeled pretty well and if you need the mig stabilised for level flight just use the AP, it works very realistic.

It's still possible to trim the mig manually, BUT make sure your airspeed is constant, otherwise of course, it's impossible to get the balance.